This invention relates to a photocathode for use in pickup tubes at very low light levels as well as to a television camera tube or an image intensifier tube.
It is known to construct a photocathode having the following main components:
a so-called window layer consisting of P.sup.+ type semiconductor in which the forbidden band is of sufficient width to ensure that said layer is transparent to the wavelengths of the light to be detected and which is bonded to a glass wall for receiving the light to be detected; PA1 a so-called absorption layer consisting of a P.sup.+ type semiconductor in which the forbidden band is of sufficiently small width to convert the light photons to be detected into electron-hole pairs; PA1 a so-called emission layer consisting of material which produces negative electron affinity at the end of the absorption layer in order to emit into vacuum the electrons which are released within the absorption layer.
In the absence of a bias applied to the absorption layer, negative electron affinity can be achieved only in the case of materials having a forbidden bandwidth greater than a predetermined limit, which imposes a limit above the detectable wavelength. A positive bias applied to the absorption layer permits the fabrication of photocathodes having good photoemissive efficiency with materials which have a smaller forbidden bandwidth and therefore absorb longer wavelengths. A bias can be applied to the absorption layer by means of a connection with said layer or by a very thin metallic electrode interposed between said layer and the emission layer. A photocathode of this type is described in the article by J. J. Escher et al., IEEE-EDL2, 123-125 (1981).
This type of photocathode has the disadvantage of high dark emission. In fact, a high hole current flows within the window layer and the absorption layer. This hole current produces electron-hole pairs within the absorption layer by ionization, thus generating a parasitic electron flow which is emitted into vacuum by the emission layer. These electrons constitute a strong background noise, which is objectionable when taking pictures at very low light levels. Moreover, the hole current is the cause of high power consumption and has the effect of heating-up the photocathode.
The aim of the invention is to produce a photocathode having a lower dark current than photocathodes of known types. The object of the invention is a photocathode having layers similar to those of the photocathode of known type but further comprising within the absorption layer one or a number of additional layers formed of semiconductor material having a forbidden band of greater width than that of the material of the absorption layer and having a thickness such that said layer or layers are practically transparent to the electron current and are practically opaque to the hole current.